This invention relates to testing the quality of a data communication channel, e.g., a telephone line linking two modems.
Testing for poor line quality not only helps to identify problems that may prevent data from being sent accurately and rapidly, but also may give a basis for a telephone company refund for lines that do not meet the specific minimum line quality standards set out in the telephone tariffs.
Modems typically are capable of monitoring customer data received over the line to give a rough qualitative indication of line performance. Armstrong, U.S. Pat. No. 4,273,955, shows including in the same chassis as the modem an auxiliary processor to test the line without interfering with transmission of customer data. The tests are directed from and the results are returned to a central network controller via an FSK channel separate from the main PSK communication channel.
One way to make the more detailed standard measurements of line quality that are specified by the tariffs is to use a testing machine and a technician at each end of the line. The line is taken out of service and the two technicians (who are in voice communication) test the line first in one direction, then in the other, using special test signals rather than customer data.
Abramson, U.S. Pat. No. 4,112,264, discloses a tester that is associated with but distinct from each modem to perform quantitative testing using special test signals in response to commands that are sent from a central site over a different transmission link (e.g., a direct distance dialed telephone line) from the data carrying transmission link.